The present invention relates to a sputtering electrode which is used in a sputtering system having a rectangular flat-plate target, which is aimed at improving uniformity of film thickness and film quality of a thin film formed on a substrate surface by sputtering, as well as for improved consumption efficiency of the sputtering target.
As compared with the vacuum deposition process, the sputtering process allows easier thin film formation of high melting point materials and compounds, but has a drawback of lower rates of thin film formation. It is the magnetron sputtering process that has eliminated this drawback and made it possible to mass-produce thin films by sputtering. The magnetron sputtering process has by now spread over a wide range of fields such as semiconductors and electronic components.
The magnetron sputtering electrode used together with a rectangular flat-plate target is one of the sputtering electrodes that are mounted on various types of sputtering systems now in a wide range of use. Included in these sputtering systems are carrousel type sputtering systems which carry out thin film formation while rotating a cylindrical substrate holder on which a large number of substrates have been mounted, large-size moving type sputtering systems which carry out thin film formation by translating relative to a target a flat-plate substrate holder on which a large number of substrates or a single large-area substrate has been placed, and other types of systems.
A conventional magnetron sputtering electrode used together with a rectangular flat-plate target is described below with reference to FIGS. 5, 6, and 7. FIG. 5 is a plan view of a conventional magnetron sputtering electrode 24 used with a rectangular flat-plate target, FIG. 6 is a sectional view taken along the line VI--VI of FIG. 5, and FIG. 7 is a perspective view of FIG. 5. Reference numeral 1 denotes a rectangular flat-plate target, which is fixed to a backing plate 2 with a soldering agent such as indium and mounted on an electrode 4 via an O-ring 3 for vacuum sealing. On the rear side of the target 1, a magnetic circuit 5 for providing magnetron discharge is arranged in such a way that closed lines of magnetic force 6 are formed and moreover at least part of the lines of magnetic force 6 become parallel to the surface of the target 1. As a result, as shown in FIG. 7, a toroidal-type, closed tunnel shaped magnetic field 7 is formed on the surface of the target 1.
The magnetron sputtering electrode 24 together with the rectangular flat-plate target 1 according to the above-described arrangement is now described with respect to its principle of operation, referring to FIGS. 7 and 9. FIG. 9 is a schematic view of a sputtering system in which the above-described sputtering electrode 24 is installed. The sputtering electrode 24 is installed in a vacuum chamber 21 via an insulating material 22, and has a dc or ac power supply 23 connected thereto. For the thin film formation, the vacuum chamber 21 is first evacuated to a high vacuum (approximately 10.sup.-7 Torr) by a vacuum pump 25.
Next, discharge gas 26 such as Ar is introduced through a flow regulator 27, and the interior of the chamber 21 is maintained at a pressure of about 10.sup.-3 to 10.sup.-2 Torr by controlling a conductance valve 28. In this state, if a negative voltage is applied to the sputtering electrode 24 with the rectangular flat-plate target 1 mounted thereon, there occurs magnetron discharge in the vicinity of the electric field and the toroidal-type tunnel-shaped magnetic field 7 of the magnetic circuit 5. Then, the target 1 is sputtered so that sputtered particles are deposited on a substrate 30 mounted on a substrate holder 29. Thus, a thin film is formed thereon.
Unfortunately, the aforementioned magnetron sputtering electrode 24 having the rectangular flat-plate target 1 has increased plasma densities at the portions thereof where the most intense lines of magnetic force pass in parallel to the surface of the target 1. Accordingly, there are formed on the target 1 areas which are sputtered (indicated by reference numeral 8 in FIG. 7, hereinafter referred to as erosion areas) and areas where sputtered particles are redeposited. This causes the target 1 to be non-uniformly eroded. Due to this, in order to ensure uniformity in thickness of thin films formed on the substrate 30 positioned opposite to the target 1, it is necessary to appropriately control the size of the target 1, the magnetic circuit, and the distance between the target 1 and the substrate 30. Generally, a target having a size about two times larger than the length of one side of the substrate is necessary to ensure uniformity of the thickness of the thin films.
Also, as the sputtering proceeds, the target 1 will vary in its erosion area 8 as well as in its configuration, so that the film thickness distribution also varies from (a) to (c) through (b) as shown in FIG. 8. This graph indicates that the film thickness tends to become thinner at the periphery of the substrate as the erosion progresses.
Furthermore, the presence of partial erosion area 8 on the target 1 would also involve non-uniformity of the physical properties of the thin films such as film composition and structure within the substrate surface or among batches, in the case of alloy sputtering or reactive sputtering.
Generally, the usage efficiency of the target 1 is as poor as 20-30%. Moreover, when the target 1 is formed of ferromagnetic material such as iron or cobalt, there are some cases where magnetic leakage fluxes onto the surface of the target 1 cannot be sufficiently obtained to implement the sputtering.